The leak of Labour’s manifesto last week was accompanied by so much political white noise that its true significance was missed. In the Blair and Brown era, the “clause V” meeting was a filter whereby any radical proposals that slipped through the party conference could be jettisoned from the manifesto. But Jeremy Corbyn turned the filter inside out: he used the meeting to make party policy on a scale, and with a rapidity, no annual conference has achieved since he took office. Labour pledged not just to reverse cuts to public services; it pledged itself to a universalist concept of the welfare state, including a Nordic-style childcare system. It pledged to renationalise railways, energy companies and the postal service. And it adopted the philosophy of taxing wealth, not just the incomes of companies and high earners, to pay for it.

The party’s rules are quite clear: all this is now Labour party policy – not some glitterball of slogans to be spun before the electorate for a few weeks and then forgotten. In a single afternoon, Labour became the first major social democratic party to jettison neoliberal economics in its entirety.

As a result, three things have changed. First, there is clarity about what Corbynism represents, both its ambitions and its limits. Faced with the choice, Corbyn opted to send 18-year-olds to university for free rather than reverse all cuts to welfare benefits. Faced with necessity, he disavowed pacifism and pledged to authorise war in the last resort.

Second, from the experience on the doorstep and on the high street this weekend, the manifesto is popular, even if Corbyn himself remains “Marmite” to some voters. Third, it accelerates the possibility of a new centre party emerging after the 8 June election.

The centrist Labour MPs trying to defend their seats on a local-only platform are doing more than simply omitting Corbyn’s name from leaflets. A script seen by the BBC tells supporters: “Admit Jeremy Corbyn won’t win. Tell voters the country needs good independent-minded MPs.” What to say about Labour’s radical manifesto pledges is not in that script – but its authors will have to come up with something. Because if, as reported, some are planning to resign the whip and go independent, there is no moral basis for doing so if they campaigned on Labour’s manifesto.

At this point you have to consider what’s happening in the liberal centre. When everybody from Conservative Anna Soubry to Vince Cable, through to those on the Progress wing of Labour is talking about the need for a new centrist party, it is logical to assume someone has a plan to form one.

I think it would be a positive development. Labour, far from collapsing like its French counterpart, has put on five percentage points in three weeks by discovering its inner Bevanite. By contrast, it is the liberal centre that has become disorganised and incoherent.

At every well-heeled political gathering, denial reflexes are stronger than coherent plans of action. Each week, a new straw emerges to clutch at: maybe the involvement of foreign tech firms makes the referendum illegal; maybe Gina Miller will work her magic in the courtroom; maybe the deal will be so bad that parliament will have to call a second referendum.

I have noticed that this denial reflex becomes stronger the further people are from having to interact with the majority of the electorate, which has accepted Brexit and wants to “make the best of it”. It is stronger among rock stars and celebrity comedians than it is, say, amongLib Dem candidates in south-west England.

The formation of a centrist party after the election would force the liberal-centrist part of civil society to focus on what it actually wants: to stop Brexit or to mitigate its impact; to keep Trident or ditch it; to fight more wars like Iraq or to forswear them?

If a new centrist party emerges, I am in favour of Labour seeking a formal alliance with it, and with the progressive nationalist parties, to oppose hard Brexit and pick up the pieces once economic chaos begins, and the victimisation of minorities starts in earnest.

There are far better examples than Macron v Mélenchon for the left to follow. In Berlin, for example, the city council is Red-Red-Green – an alliance between the social democrats, the former communists and the Greens. They don’t like each other, but they have to work together and negotiate a platform through give and take. It is much better to do it this way than the “winner takes all” battle that led to Labour’s new manifesto.

For the forseeable future, progressive politics in Britain will be a story of alliances. Those inside Labour who cannot stomach the decisive rejection of free-market economics and adventurist war should have the decency to campaign on the manifesto, which was democratically scrutinised and agreed.

They should be proud of Labour’s pledges to end the health cuts, make university tuition free, end the elderly care crisis and begin the move to a Nordic model of childcare. If they can’t be, they will feel happier elsewhere.

Like tens of thousands of party activists, I feel this is a manifesto Labour could win on, even if it may not in the extraordinary circumstances of 2017. It has taken my generation of the left half a lifetime to end Labour’s addiction to free-market capitalism and expeditionary warfare. We are not going back.